Increasing oil prices and national legislation requiring the reduction of automotive carbon dioxide emissions force tire and rubber producers to produce “fuel-efficient” and thus fuel-saving tires. One general approach to obtain fuel-efficient tires is to produce tire formulations which have reduced hysteresis loss. A major source of hysteresis in vulcanized elastomeric polymers is attributed to free polymer chain ends, i.e. the section of the elastomeric polymer chain between the last cross-link and the end of the polymer chain. This free end of the polymer does not participate in the efficient elastically recoverable process and, as a result, energy transmitted to this section of the polymer is lost. The dissipated energy leads to a pronounced hysteresis under dynamic deformation. Another source of hysteresis in vulcanized elastomeric polymers is attributed to an insufficient distribution of filler particles in the vulcanized elastomeric polymer composition. The hysteresis loss of a cross-linked elastomeric polymer composition is related to its tan δ value at 60° C. (see ISO 4664-1:2005; Rubber. Vulcanized or thermoplastic; Determination of dynamic properties—part 1: General guidance). In general, vulcanized elastomeric polymer compositions having relatively small tan δ values at 60° C. are preferred as having lower hysteresis loss. In the final tire product, this translates into a lower rolling resistance and better fuel economy.
It is generally accepted that a lower rolling resistance tire can be made at the expense of deteriorated wet grip properties. For example, if, in a random solution styrene-butadiene rubber (random SSBR), the polystyrene unit concentration is reduced with respect to the total polybutadiene unit concentration, and the 1,2-polybutadiene unit concentration is kept constant, the SSBR glass transition temperature is reduced and, as a result, both tan δ at 60° C. and tan δ at 0° C. are reduced, generally corresponding to improved rolling resistance and deteriorated wet grip performance of the tire. Similarly, if, in a random SSBR, the 1,2-polybutadiene unit concentration is reduced with respect to the total polybutadiene unit concentration, and the polystyrene unit concentration is kept constant, the SSBR glass transition temperature is reduced and, as a result, both tan δ at 60° C. and tan δ at 0° C. are reduced, generally corresponding to improved rolling resistance and deteriorated wet grip performance of the tire. Accordingly, when assessing the rubber vulcanizate performance correctly, both the rolling resistance, related to tan δ at 60° C., and the wet grip, related to tan δ at 0° C., should be monitored along with the tire heat build-up.
One generally accepted approach for reducing hysteresis loss is to reduce the number of free chain ends of elastomeric polymers. Various techniques have been described in the literature, including the use of “coupling agents” such as tin tetrachloride, which may functionalize the polymer chain end and react with components of an elastomeric composition, for example with a filler or with unsaturated portions of a polymer. Examples of such techniques and coupling agents are described in the following patents: U.S. Pat. Nos. 3,281,383; 3,244,664 and 3,692,874 (for example, tetrachlorosilane); U.S. Pat. Nos. 3,978,103; 4,048,206; 4,474,908; 6,777,569 (blocked mercaptosilanes) and U.S. Pat. No. 3,078,254 (a multi-halogen-substituted hydrocarbon, such as 1,3,5-tri(bromomethyl)benzene); U.S. Pat. No. 4,616,069 (tin compounds and organic amino or amine compounds); and U.S. 2005/0124740.
The reference article “Synthesis of end-functionalized polymer by means of living anionic polymerization”, Journal of Macromolecular Chemistry and Physics, 197, (1996), 3135-3148, describes the synthesis of “polystyrene-containing” and “polyisoprene-containing” living polymers with hydroxy (—OH) and mercapto (—SH) functional end caps, obtained by reaction of living polymers with haloalkanes containing silyl ether and silyl thioether functions. The tertiary-butyldimethylsilyl (TBDMS) group is preferred as a protecting group for the —OH and —SH functional groups in the termination reactions, because the corresponding silyl ethers and thioethers are found to be both stable and compatible with anionic living polymers.
WO 2007/047943 describes the use of a silane sulfide omega chain end modifier represented by the formula (RO)x(R)ySi—R′—S—SiR3, wherein x is 1, 2 or 3, y is 0, 1 or 2, x+y=3, R is alkyl, and R′ is aryl, alkylaryl or alkyl, to produce a chain end-modified elastomeric polymer, which is used as component in a vulcanized elastomeric polymer composition or a tire tread.
More specifically, according to WO 2007/047943, a silane sulfide compound is reacted with anionically-initiated living polymers to produce “chain end-modified” polymers, which are subsequently blended with fillers, vulcanizing agents, accelerators or oil extenders to produce a vulcanized elastomeric polymer composition having low hysteresis loss.
The vulcanized elastomeric polymer compositions are described as exhibiting lower tan δ values at 60° C., particularly in comparison with compounds based on corresponding non-modified polymers, without negatively affecting tan δ values at 0° C. and processing characteristics. Individual exemplary cured polymer formulations comprising modified polymers are shown to result in reduced tan δ at 60° C. and heat build-up values but equivalent tan δ values at 0° C. They are described as being useful in preparing tire treads having lower rolling resistance, while maintaining good wet grip properties. In case the modifier contains two or three alkoxy groups (x=2 or 3), the resulting functionalized polymer comprises —Si—OR groups and —S—SiR3 groups, which are converted under suitable conditions, such as typically present during reactive mixing of functionalized polymers with fillers, into silanol groups (—Si—OH) and thiol groups (—S—H). Silanol groups and thiol groups are reactive with respect to fillers containing silanol surface groups, such as silica. Thus, the formation of bonds between functionalized polymer and silica is expected. Although cured rubber hysteresis properties can be improved significantly through application of the technology described in WO 2007/047943, the impact of the technology is limited due to the fact that only one polymer chain end can be functionalized by using the modifier compound described. Accordingly, there is a need for an efficient modification of the second polymer chain end.
There is a need for modification methods and resulting polymers, including modified polymers, which can be used for further optimizing dynamic properties of vulcanizates containing silica and carbon black, including low hysteresis loss and high abrasion resistance, corresponding to a high wet grip, low rolling resistance and high abrasion resistance in tires. In addition, there is a need to further decrease the vulcanizate heat build-up during thermal exposure and under mechanical stress. These needs have been met by the following invention.